1. Field of the Invention
This invention relates to welding materials for high-Cr steels which are suitable for use in boilers and turbines for electric power generation, chemical plants, and the like.
2.Description of the Related Art
When welding materials for high-Cr steels which have been developed up to now are used for welding purposes, the toughness of the weld metal is reduced. Consequently, in order to secure high toughness, welded joints formed by gas tungsten-arc welding (hereinafter abbreviated as GTAW) or gas metal-arc welding (hereinafter abbreviated as GMAW) must be subjected to a postweld heat treatment at 740-760.degree. C.
However, where a high-Cr steel is used in combination with a low-Cr steel material (e.g., 21/4Cr-1Mo steel) for which a postweld heat treatment at a high temperature of 740-760.degree. C. cannot be employed, it is conventional practice to first subject the high-Cr steel to a postweld heat treatment at 740-760.degree. C. and then subject the 21/4Cr-1Mo steel, together with the high-Cr steel, to a postweld heat treatment at 700-730.degree. C.
Accordingly, in order to overcome the disadvantage of requiring two-stage heat treatment, the present applicant has proposed, in Japanese Patent Provisional Publication No. 8-290290, a high-toughness welding material for high-Cr steels which permits high-Cr steels welded by GTAW and GMAW to be subjected to a postweld heat treatment at 700-730.degree. C. similarly to 21/4Cr-1Mo steel, and thereby makes it possible to simplify the production process and save thermal energy.
Specifically, the welding material proposed in the aforementioned patent is a high-toughness welding material for high-Cr steels which is suitable for use in the gas tungsten-arc welding or gas metal-arc welding of high-Cr steels. Its chemical composition contains, on a weight percentage basis, up to 0.1% C, up to 0.3% Si, 0.2 to 1.5% Mn, up to 0.02% P, up to 0.01% S, 8 to 13% Cr, up to 0.75% Ni, 0.5 to 3% Mo, 0.18 to 0.25% V, 0.05 to 0.3% Ta, and 0.002 to 0.005% B, the balance being Fe.
In this welding material, Nb which has been contained in conventional welding materials for the purpose of enhancing creep rupture strength is replaced with Ta, and an appropriate amount of B is added, so as to strengthen grain boundaries and enhance long-time creep rupture strength and toughness. This makes it possible to perform the postweld heat treatment in a single stage instead of performing it in two stages using different temperatures, and thereby decrease the number of process steps and afford a saving of energy.
Recently, in order to improve the thermal efficiency of thermal electric power plants, materials having better temperature characteristics are being developed by using the composition of a conventional high-Cr steel as a basic composition and adding W, N and/or Co thereto, and are being put to practical use.
However, when such a homologous welding material using the composition of a high-Cr steel as a basic composition or a quasi-homologous welding materials obtained by replacing Nb contained in the basis composition with Ta and adding an appropriate amount of B is used for welding purposes, the toughness of the weld metal is markedly reduced and, therefore, cannot be satisfactorily restored by a postweld heat treatment at 700-730.degree. C. Accordingly, where it is desired to use a high-Cr steel is in combination with 21/4Cr-1Mo steel, a structure of high-Cr steel is subjected to a postweld heat treatment at 740-760.degree. C., while a structure of 21/4Cr-1Mo steel is subjected to a postweld heat treatment at 700-730.degree. C. Thereafter, the aforesaid structure of high-Cr steel and the aforesaid structure of 21/4Cr-1Mo steel are welded together, and the resulting heterogeneous welded joint is locally subjected to a postweld heat treatment at 700-730.degree. C. Alternatively, the aforesaid structure of high-Cr steel is subjected to a postweld heat treatment at 740-760.degree. C. and then welded to a structure of 21/4Cr-1Mo steel. Thereafter, the resulting integral structure consisting of the structure of high-Cr steel and the structure of 21/4Cr-1Mo steel is subjected to a postweld heat treatment at 700-730.degree. C. As used herein, the terms "homologous welding material" and "quasi-homologous welding material" mean welding materials in which the principal components are the same as those of the base metal.